The present invention is generally directed to the production of Group III metal nitride materials for use as free-standing articles as well as substrates for further processes and/or microelectronic and optoelectronic devices. In particular, the present invention is directed to the production of low-defect density, single-crystal materials and highly-oriented polycrystalline materials utilizing enhanced sputtering techniques.
A wide variety of techniques exist for depositing thin films onto substrates in order to achieve desirable properties which are either different from, similar to, or superior to the properties of the substrates themselves. Thin films are employed in many kinds of optical, electrical, magnetic, chemical, mechanical and thermal applications. Optical applications include reflective/anti-reflective coatings, interference filters, memory storage in compact disc form, and waveguides. Electrical applications include insulating, conducting and semiconductor devices, as well as piezoelectric drivers. Magnetic applications include memory discs. Chemical applications include barriers to diffusion or alloying (e.g., galling), protection against oxidation or corrosion, and gas or liquid sensors. Mechanical applications include tribological (wear-resistant) coatings, materials having desirable hardness or adhesion properties, and micromechanics. Thermal applications include barrier layers and heat sinks.
Bulk materials can be used as substrates upon which microelectronic and optical devices are fabricated. Wide bandgap semiconductor materials such as gallium nitride, aluminum nitride, indium nitride and their alloys are being studied for their potential application in microelectronics and opto-electronics. These materials are particularly well suited for short wavelength optical applications, such as green, blue and UV light emitting devices (LEDs and LDs), and visible and solar-blind UV detectors. The use of UV or blue GaN-based LEDs makes possible the fabrication of solid state white light sources, with higher efficiencies and lifetimes 10 to 100 times longer than conventional sources. Additionally, GaN has a region of negative differential mobility with a high peak electron velocity and high-saturated velocity, which can be used for fabricating high-speed switching and microwave components. P-type doping of GaN and AlGaN with relatively high hole concentrations is now readily achieved, and ohmic and Schottky contacts have been characterized for n- and p-type materials. Thus, many of the above devices have or potentially have large, technologically important markets. Such markets include display technology, optical storage technology, and space-based communications and detection systems. Other applications include high temperature microelectronics, opto-electronic devices, piezoelectric and acousto-optic modulators, negative-electron affinity devices and radiation/EMP hard devices for military and space uses.
Attempts to grow low-defect density gallium nitride (GaN) thin films heteroepitaxially on substrates such as sapphire and silicon carbide (SiC) have had limited success. GaN materials heteroepitaxially grown on these substrates suffer from large concentrations of threading defects, typically on the order of 10xe2x88x928xe2x88x9210xe2x88x9210 cmxe2x88x922, due to the large lattice mismatch between the film and substrate. Threading defects increase leakage currents in diode and FET structures and act as a significant source of noise in photodetectors. As a result, the operation of high performance devices, such as high-speed, high-sensitivity UV photodetectors, and high power, high frequency microelectronic devices, is presently limited. Buffer layers of AIN, GaN, and other materials have been used to reduce the lattice mismatch. However, threading defects and low angle grain boundaries remain in the films. Differences between the film and substrate thermal expansion coefficients also result in stresses in the films.
Accordingly, homoepitaxial growth of GaN thin films on bulk GaN substrates is of great interest. The use of GaN substrates would eliminate the problems due to lattice mismatch and thermal expansion mismatch. Unfortunately, the availability of GaN substrates has been limited due to conventional processing capabilities. This problem has hindered the development of devices based on GaN and related nitride semiconductors. Several obstacles exist to the successful manufacturing and commercializing of high device-quality Group III nitride-based materials, whether in bulk, single-crystal, polycrystalline or epitaxial form, for electronics and other applications. These obstacles generally include cost, reproducibility, and purity.
For instance, gallium nitride has a high equilibrium vapor pressure of nitrogen that results in its decomposition at elevated temperatures. The solubility of nitrogen in gallium metal at room temperature and pressure is very low. As a result, conventional crystal growth methods to produce GaN are not practical. This has led to the development of several alternate bulk growth methods, including high-temperature, high-pressure (15 kbar) solution growth, evaporation, and sublimation.
Currently, aluminum nitride and gallium nitride exist only as polycrystalline or powder forms, or in thin films. Polycrystalline bulk aluminum nitride can be manufactured using powder processing techniques. This process has not yielded semiconductor-grade single crystal material. Formidable problems are associated with such techniques, beginning with the production of pure aluminum nitride powders and then the sintering of oxygen-free and defect-free aluminum nitride. Some of these problems include the production of both high-purity and uniform particle-size powders. The highest purity powders can contain up to 1% of oxygen and binders, such as Y2O3, that are needed to produce aluminum nitride with a high density. Therefore, high density is achievable at the expense of contamination. Sintering of these aluminum nitride powders is also a difficult process. The covalent nature of aluminum nitride prevents densification of pure aluminum nitride at low temperatures. Aluminum nitride decomposes at high temperatures, such as above 1600xc2x0 C., thereby preventing densification. Hence, costly sintering aids such as high pressures and impurities are required for producing high-density material. Other problems associated with powder processing of aluminum nitride include maintaining the purity and integrity of the powder, controlling the environment at high sintering temperatures, and the production of defect-free parts. Aluminum nitride is very difficult to manufacture using powder processing techniques without introducing contamination that will have adverse effects on the optical and thermal properties of the material. These impurities can be present in the crystalline lattice structure, and can migrate to the grain boundaries during sintering, causing the infrared absorbance to be high.
As disclosed hereinbelow, it has now been discovered that enhanced sputtering techniques, which are physical vapor deposition (PVD) techniques, can be feasibly utilized to produce low-defect density Group III metal nitride materials of bulk thickness and of device-quality crystal. Magnetron sputtering is traditionally associated with thin film deposition. An advantage of sputter synthesis is that high purity compounds can be formed directly from the high purity source materials. Moreover, the synthesis can be achieved under highly controlled conditions. Nitrogen and Group III metals such as aluminum are readily available, from multiple sources, in ultra-high purity grades (e.g., 99.9999%) for the microelectronics industry. Sputter synthesis is currently the process that most effectively eliminates hydrogen from the bulk, since the sputter environment is controllable to ultra-high vacuum conditions. Through sputter synthesis of Group III nitrides, it is possible to obtain materials that have properties near the bulk properties. Since this takes place under ultra-high vacuum conditions, hydrogen and oxygen can be eliminated from the material. Reactive sputtering has the advantage of producing high purity materials with high densities, and ease of fabrication of quality crystalline material.
However, traditional magnetron sputtering has several drawbacks, which has made it very difficult to produce bulk materials. These drawbacks include unwanted target reactions, transport limitations, and low growth rates. During reactive magnetron sputtering, micro-arcs can occur on the cathode surface which cause imperfections in the deposited material. Another problem associated with this process is the xe2x80x9cdisappearing anodexe2x80x9d effect, in which the entire anode becomes covered by randomly grown insulating layers. Also related to this process is the problematic formation of an insulating nitride layer on the target surface that increases the impedance of the cathode until the target becomes xe2x80x9cpoisonedxe2x80x9d or completely insulating. This results in a drastic decrease in deposition rates to almost zero when the target becomes too nitrided to operate. Materials transport can also be a problem in bulk crystal growth using magnetron sputtering since there can be a significant loss of material to the sidewalls.
The present invention is provided to address these and other problems associated with the growth of thin films and bulk materials.
According to one method of the present invention, a single-crystal MIIIN article is produced. A template material having an epitaxial-initiating growth surface is provided. A Group III metal target is sputtered in a plasma-enhanced environment to produce a Group III metal source vapor. The Group III metal source vapor is combined with a nitrogen-containing gas to produce reactant vapor species comprising the Group III metal and nitrogen. The reactant vapor species is deposited on the growth surface to produce a single-crystal MIIIN lever thereon. In one aspect, the single-crystal MIIIN layer is grown as a thin film. i.e., with a thickness of approximately 10 to 10,000 mn (0.01 to 10 microns), for use as a seed crystal upon which a bulk, second MIIIN layer can be grown. In another aspect growth of the MIIIN layer is permitted to continue beyond the thin film range until its thickness is sufficient to ensure that the resulting bulk crystal has a low enough defect density to be considered as device-quality. In a further aspect, the MIIIN layer is grown to a bulk thickness and the template material is removed, thereby providing a free-standing, single-crystal MIIIN article having a diameter of approximately 0.5 inch or greater and a thickness of approximately 50 microns or greater.
Methods of the present invention can be implemented by providing a novel sputter material transport device to enhance thin-film and bulk material manufacturing processes. The novel transport device is capable of ultra-high deposition and growth rates, making it feasible for growing thick material and increasing throughput in manufacturing processes. The transport device can be used both to grow bulk crystalline materials and to deposit thin films and epitaxial layers onto bulk substrates. Generally, as compared to other sputter processes, the transport device has the advantages of lowered processing pressure, higher deposition rates, higher ionization efficiency, and a controlled processing environment with no contamination. The transport device utilizes an enhanced sputtering process to rapidly deposit both metallic and dielectric materials. This enhancement allows the process to overcome the limitations of conventional PVD techniques.
The transport device according to the present invention can achieve growth rates in excess of ten times those achieved by any other direct deposition process. As currently tested, the device is capable of depositing single or polycrystalline material at a rate in excess of approximately 60 xcexcm/hr. This high deposition rate allows for high throughput capabilities and the possibility of manufacturing bulk materials in short time periods. The device has increased growth rates due to the very high ionization efficiencies, which enhances the sputtering process without xe2x80x9cpoisoningxe2x80x9d the sputtering material. The ability to deposit material at high deposition rates will have many commercial applications, including high-throughput manufacturing processes of thick films of exotic materials. Moreover, high-quality material can be deposited in a cost-effective manner. It is also projected that the device will aid in the commercialization of bulk dielectric and semiconductor materials and will have numerous applications to other materials.
The transport device surpasses present technology by offering a non-contaminating method, in the form of a triode sputtering arrangement, to increase the ionization efficiency and hence the overall deposition rate. The device also has the advantage of a cooler operating temperature than a thermionic hollow cathode configuration, allowing the injector means of the device to be composed of low-temperature materials, and thus can apply to a broad range of materials as compared to conventional processes. The transport device can increase the deposition rate of the target material and lower the sputtering pressure, thereby enabling a line-of-sight deposition process.
The transport device is capable of growing bulk material such as aluminum nitride and other Group III nitrides and also is capable of depositing metal in deep trenches for the semiconductor industry.
According to the present invention, the transport device includes a magnetron source and a non-thermionic electron (or, in effect, a plasma) injector assembly to enhance magnetron plasma. Preferably, the electron/plasma injector is disposed just below the surface of a cathode target material, and includes a plurality of non-thermionic, hollow cathode-type injector devices for injecting electrons into a magnetic field produced by a magnetron source. The injector can be scaled in a variety of configurations (e.g., circular or linear) to accommodate various magnetron shapes. When provided in the form of a circular ring, the injector includes multiple hollow cathodes located around the inner diameter of the ring.
The transport device constitutes an improvement over previously developed hollow cathode enhanced magnetron sputtering devices that rely on thermionic emission. The device of the present invention comprises a non-thermionic electron emitter that operates as a xe2x80x9ccoldxe2x80x9d plasma source and can be composed of the same material as its sputtering target. The injector can be manufactured out of high-purity metals (e.g., 99.9999%), thereby eliminating a source of contamination in the growing film. The addition of the injector to the magnetron sputtering process allows higher deposition rates as compared to rates previously achieved by conventional magnetron sputtering devices. Moreover, the transport device takes advantage of the hollow cathode effect by injecting electrons and plasma into the magnetic field to increase plasma densities without the contamination problem associated with a traditional, thermionic-emitting tantalum tip. As disclosed above, the transport device is further characterized by a decreased operating pressure and an increased ionization rate over conventional magnetron sputtering.
Therefore, according to another method of the present invention, a single-crystal MIIIN article is produced by using a sputtering apparatus comprising a non-thermionic electron/plasma injector assembly to produce the Group III metal source vapor from a Group III metal target. The Group III metal source vapor is combined with a nitrogen-containing gas to produce reactant vapor species comprising Group III metal and nitrogen. The reactant vapor species is deposited on the growth surface of the template material to produce a single-crystal MIIIN layer thereon.
The sputter transport device comprises a sealable or evacuable, pressure controlled chamber defining an interior space, a target cathode disposed in the chamber, and a substrate holder disposed in the chamber and spaced at a distance from the target cathode. The target cathode is preferably bonded to a target cathode holder and negatively biased. A magnetron assembly is disposed in the chamber proximate to the target cathode. A negatively-biased, non-thermionic electron/plasma injector assembly is disposed between the target cathode and the substrate holder. The injector assembly fluidly communicates with a reactive gas source and includes a plurality of hollow cathode-type structures. Each hollow cathode includes an orifice communicating with the interior space of the chamber.
According to one aspect of the present invention, the electron/plasma injector assembly is adapted for non-thermionically supplying plasma to a reaction chamber. The injector assembly comprises a main body and a plurality of replaceable or interchangeable gas nozzles. The main body has a generally annular orientation with respect to a central axis, and includes a process gas section and a cooling section. The process gas section defines a process gas chamber and the cooling section defines a heat transfer fluid reservoir. The gas nozzles are removably disposed in the main body in a radial orientation with respect to the central axis and in heat transferring relation to the heat transfer fluid reservoir. Each gas nozzle provides fluid communication between the process gas chamber and the exterior of the main body.
The methods of the present invention can be utilized to successfully produce device-quality articles.
According to one embodiment of the present invention, a bulk single-crystal MIIIN article has a diameter of approximately 0.5 inch to approximately 12 inches and a thickness of approximately 50 microns or greater.
According to another embodiment of the present invention, a single-crystal MIIIN article is produced in wafer form, having a thickness ranging from approximately 50 microns to approximately 1 mm.
According to yet another embodiment of the present invention, a single-crystal MIIIN article is produced in boule form, having a diameter of approximately 2 inches or greater and a thickness ranging from approximately 1 mm to greater than approximately 100 mm.
According to still another embodiment of the present invention, the single-crystal MIIIN layer is used as a seed crystal, such that additional reactant vapor species comprising the Group III metal and nitrogen can be deposited the MIIIN layer to produce a bulk, homoepitaxially grown MIIIN article.
In conjunction with the methods of the present invention wherein a bulk MIIIN article is produced, a wafer can be cut from the MIIIN article and an epitaxial layer subsequently deposited on the wafer.
The single-crystal MIIIN layers or articles produced according to the methods of the present invention can be formed at a growth rate greater than approximately 10 microns/hour.
In conjunction with the methods of the present invention, microelectronic or optoelectronic devices or components can be fabricated on the MIIIN layers or articles, or on any additional layer grown on the MIIIN layers or articles.
According to a further embodiment of the present invention, a highly-oriented polycrystalline Group III nitride material is provided. The material has an elongate surface and a plurality of grain boundaries oriented substantially normal to the elongate surface. Thermal conductivity is high (i.e., promoted or enhanced) through the thickness of the material in a direction substantially normal to the elongate surface, and is low (i.e., impeded) in a direction substantially parallel to the elongate surface. The material is transparent to radiative energy in the infrared spectrum, the microwave spectrum, or both spectra, along the direction substantially normal to the elongate surface. As part of the growth process of the material, the material can be bonded to a metallic frame and employed in applications in which its directional thermal conductivity and/or its transparency is advantageous.
According to a further method of the present invention, a window is produced that is adapted to transmit radiative energy in the infrared and/or microwave spectra. A negatively-biased target cathode including a target material is provided in a sealed chamber. A metallic frame is provided in the chamber and spaced at a distance from the target cathode. An operating voltage is applied to the target cathode to produce an electric field within the chamber. A magnetron assembly is provided in the chamber to produce a magnetic field within the chamber. A negatively-biased, non-thermionic electron/plasma injector assembly is provided between the target cathode and the metallic frame to create an intense plasma proximate to the target cathode. A background gas is introduced into the chamber to provide an environment for generating a plasma medium. A portion of the target material is sputtered and transported through the plasma medium toward the metallic frame.
It is therefore an object of the present invention to provide low-defect density, single-crystal Group III nitride articles, substrates and device layers characterized by purities and sizes that previously have been unattainable.
It is another object of the present invention to provide a novel sputter material transport method and device capable of ultra-high deposition and growth rates of low-defect density Group III nitride materials.
It is a further object of the present invention to provide a polycrystalline material in bulk form which can transmit infrared and/or microwave energy.
Some of the objects of the invention having been stated hereinabove, other objects will become evident as the description proceeds when taken in connection with the accompanying drawings as best described hereinbelow.